Update without interuption

Is there any other way to download iOS 6 into the computer, so that i can update it later on? I don't want to wait 3-4 hrs updating my iPhone. abt 650mb file is pretty big to download in my case. Also since there is irregular interuption with my net, i cannot trust to let my iTune syc it within 3-4 hrs.
I don't think that i can update half the file today, and rest later on from iTune either.